[for-next][PATCH 6/8] Documentation/rtla: Add timerlat-top auto-analysis options

From: Steven Rostedt
Date: Sat Feb 18 2023 - 15:19:58 EST

From: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>

Add the new options to the man page, as well as updating the
example to include the new output.

Link: https://lkml.kernel.org/r/a3f5fb49432a55c3323b18725fc6e702f16ccc79.1675179318.git.bristot@xxxxxxxxxx

Cc: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>
Cc: Jonathan Corbet <corbet@xxxxxxx>
Signed-off-by: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>
Signed-off-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
---
.../tools/rtla/common_timerlat_aa.rst | 7 +
.../tools/rtla/rtla-timerlat-top.rst | 164 ++++++++----------
2 files changed, 80 insertions(+), 91 deletions(-)
create mode 100644 Documentation/tools/rtla/common_timerlat_aa.rst

diff --git a/Documentation/tools/rtla/common_timerlat_aa.rst b/Documentation/tools/rtla/common_timerlat_aa.rst
new file mode 100644
index 000000000000..077029e6b289
--- /dev/null
+++ b/Documentation/tools/rtla/common_timerlat_aa.rst
@@ -0,0 +1,7 @@
+**--dump-tasks**
+
+ prints the task running on all CPUs if stop conditions are met (depends on !--no-aa)
+
+**--no-aa**
+
+ disable auto-analysis, reducing rtla timerlat cpu usage
diff --git a/Documentation/tools/rtla/rtla-timerlat-top.rst b/Documentation/tools/rtla/rtla-timerlat-top.rst
index 7c4e4b109493..73799c1150ad 100644
--- a/Documentation/tools/rtla/rtla-timerlat-top.rst
+++ b/Documentation/tools/rtla/rtla-timerlat-top.rst
@@ -30,102 +30,84 @@ OPTIONS

.. include:: common_options.rst

+.. include:: common_timerlat_aa.rst
+
EXAMPLE
=======

-In the example below, the *timerlat* tracer is set to capture the stack trace at
-the IRQ handler, printing it to the buffer if the *Thread* timer latency is
-higher than *30 us*. It is also set to stop the session if a *Thread* timer
-latency higher than *30 us* is hit. Finally, it is set to save the trace
-buffer if the stop condition is hit::
+In the example below, the timerlat tracer is dispatched in cpus *1-23* in the
+automatic trace mode, instructing the tracer to stop if a *40 us* latency or
+higher is found::

- [root@alien ~]# rtla timerlat top -s 30 -T 30 -t
- Timer Latency
- 0 00:00:59 | IRQ Timer Latency (us) | Thread Timer Latency (us)
+ # timerlat -a 40 -c 1-23 -q
+ Timer Latency
+ 0 00:00:12 | IRQ Timer Latency (us) | Thread Timer Latency (us)
CPU COUNT | cur min avg max | cur min avg max
- 0 #58634 | 1 0 1 10 | 11 2 10 23
- 1 #58634 | 1 0 1 9 | 12 2 9 23
- 2 #58634 | 0 0 1 11 | 10 2 9 23
- 3 #58634 | 1 0 1 11 | 11 2 9 24
- 4 #58634 | 1 0 1 10 | 11 2 9 26
- 5 #58634 | 1 0 1 8 | 10 2 9 25
- 6 #58634 | 12 0 1 12 | 30 2 10 30 <--- CPU with spike
- 7 #58634 | 1 0 1 9 | 11 2 9 23
- 8 #58633 | 1 0 1 9 | 11 2 9 26
- 9 #58633 | 1 0 1 9 | 10 2 9 26
- 10 #58633 | 1 0 1 13 | 11 2 9 28
- 11 #58633 | 1 0 1 13 | 12 2 9 24
- 12 #58633 | 1 0 1 8 | 10 2 9 23
- 13 #58633 | 1 0 1 10 | 10 2 9 22
- 14 #58633 | 1 0 1 18 | 12 2 9 27
- 15 #58633 | 1 0 1 10 | 11 2 9 28
- 16 #58633 | 0 0 1 11 | 7 2 9 26
- 17 #58633 | 1 0 1 13 | 10 2 9 24
- 18 #58633 | 1 0 1 9 | 13 2 9 22
- 19 #58633 | 1 0 1 10 | 11 2 9 23
- 20 #58633 | 1 0 1 12 | 11 2 9 28
- 21 #58633 | 1 0 1 14 | 11 2 9 24
- 22 #58633 | 1 0 1 8 | 11 2 9 22
- 23 #58633 | 1 0 1 10 | 11 2 9 27
- timerlat hit stop tracing
- saving trace to timerlat_trace.txt
- [root@alien bristot]# tail -60 timerlat_trace.txt
- [...]
- timerlat/5-79755 [005] ....... 426.271226: #58634 context thread timer_latency 10823 ns
- sh-109404 [006] dnLh213 426.271247: #58634 context irq timer_latency 12505 ns
- sh-109404 [006] dNLh313 426.271258: irq_noise: local_timer:236 start 426.271245463 duration 12553 ns
- sh-109404 [006] d...313 426.271263: thread_noise: sh:109404 start 426.271245853 duration 4769 ns
- timerlat/6-79756 [006] ....... 426.271264: #58634 context thread timer_latency 30328 ns
- timerlat/6-79756 [006] ....1.. 426.271265: <stack trace>
- => timerlat_irq
- => __hrtimer_run_queues
- => hrtimer_interrupt
- => __sysvec_apic_timer_interrupt
- => sysvec_apic_timer_interrupt
- => asm_sysvec_apic_timer_interrupt
- => _raw_spin_unlock_irqrestore <---- spinlock that disabled interrupt.
- => try_to_wake_up
- => autoremove_wake_function
- => __wake_up_common
- => __wake_up_common_lock
- => ep_poll_callback
- => __wake_up_common
- => __wake_up_common_lock
- => fsnotify_add_event
- => inotify_handle_inode_event
- => fsnotify
- => __fsnotify_parent
- => __fput
- => task_work_run
- => exit_to_user_mode_prepare
- => syscall_exit_to_user_mode
- => do_syscall_64
- => entry_SYSCALL_64_after_hwframe
- => 0x7265000001378c
- => 0x10000cea7
- => 0x25a00000204a
- => 0x12e302d00000000
- => 0x19b51010901b6
- => 0x283ce00726500
- => 0x61ea308872
- => 0x00000fe3
- bash-109109 [007] d..h... 426.271265: #58634 context irq timer_latency 1211 ns
- timerlat/6-79756 [006] ....... 426.271267: timerlat_main: stop tracing hit on cpu 6
-
-In the trace, it is possible the notice that the *IRQ* timer latency was
-already high, accounting *12505 ns*. The IRQ delay was caused by the
-*bash-109109* process that disabled IRQs in the wake-up path
-(*_try_to_wake_up()* function). The duration of the IRQ handler that woke
-up the timerlat thread, informed with the **osnoise:irq_noise** event, was
-also high and added more *12553 ns* to the Thread latency. Finally, the
-**osnoise:thread_noise** added by the currently running thread (including
-the scheduling overhead) added more *4769 ns*. Summing up these values,
-the *Thread* timer latency accounted for *30328 ns*.
-
-The primary reason for this high value is the wake-up path that was hit
-twice during this case: when the *bash-109109* was waking up a thread
-and then when the *timerlat* thread was awakened. This information can
-then be used as the starting point of a more fine-grained analysis.
+ 1 #12322 | 0 0 1 15 | 10 3 9 31
+ 2 #12322 | 3 0 1 12 | 10 3 9 23
+ 3 #12322 | 1 0 1 21 | 8 2 8 34
+ 4 #12322 | 1 0 1 17 | 10 2 11 33
+ 5 #12322 | 0 0 1 12 | 8 3 8 25
+ 6 #12322 | 1 0 1 14 | 16 3 11 35
+ 7 #12322 | 0 0 1 14 | 9 2 8 29
+ 8 #12322 | 1 0 1 22 | 9 3 9 34
+ 9 #12322 | 0 0 1 14 | 8 2 8 24
+ 10 #12322 | 1 0 0 12 | 9 3 8 24
+ 11 #12322 | 0 0 0 15 | 6 2 7 29
+ 12 #12321 | 1 0 0 13 | 5 3 8 23
+ 13 #12319 | 0 0 1 14 | 9 3 9 26
+ 14 #12321 | 1 0 0 13 | 6 2 8 24
+ 15 #12321 | 1 0 1 15 | 12 3 11 27
+ 16 #12318 | 0 0 1 13 | 7 3 10 24
+ 17 #12319 | 0 0 1 13 | 11 3 9 25
+ 18 #12318 | 0 0 0 12 | 8 2 8 20
+ 19 #12319 | 0 0 1 18 | 10 2 9 28
+ 20 #12317 | 0 0 0 20 | 9 3 8 34
+ 21 #12318 | 0 0 0 13 | 8 3 8 28
+ 22 #12319 | 0 0 1 11 | 8 3 10 22
+ 23 #12320 | 28 0 1 28 | 41 3 11 41
+ rtla timerlat hit stop tracing
+ ## CPU 23 hit stop tracing, analyzing it ##
+ IRQ handler delay: 27.49 us (65.52 %)
+ IRQ latency: 28.13 us
+ Timerlat IRQ duration: 9.59 us (22.85 %)
+ Blocking thread: 3.79 us (9.03 %)
+ objtool:49256 3.79 us
+ Blocking thread stacktrace
+ -> timerlat_irq
+ -> __hrtimer_run_queues
+ -> hrtimer_interrupt
+ -> __sysvec_apic_timer_interrupt
+ -> sysvec_apic_timer_interrupt
+ -> asm_sysvec_apic_timer_interrupt
+ -> _raw_spin_unlock_irqrestore
+ -> cgroup_rstat_flush_locked
+ -> cgroup_rstat_flush_irqsafe
+ -> mem_cgroup_flush_stats
+ -> mem_cgroup_wb_stats
+ -> balance_dirty_pages
+ -> balance_dirty_pages_ratelimited_flags
+ -> btrfs_buffered_write
+ -> btrfs_do_write_iter
+ -> vfs_write
+ -> __x64_sys_pwrite64
+ -> do_syscall_64
+ -> entry_SYSCALL_64_after_hwframe
+ ------------------------------------------------------------------------
+ Thread latency: 41.96 us (100%)
+
+ The system has exit from idle latency!
+ Max timerlat IRQ latency from idle: 17.48 us in cpu 4
+ Saving trace to timerlat_trace.txt
+
+In this case, the major factor was the delay suffered by the *IRQ handler*
+that handles **timerlat** wakeup: *65.52%*. This can be caused by the
+current thread masking interrupts, which can be seen in the blocking
+thread stacktrace: the current thread (*objtool:49256*) disabled interrupts
+via *raw spin lock* operations inside mem cgroup, while doing write
+syscall in a btrfs file system.
+
+The raw trace is saved in the **timerlat_trace.txt** file for further analysis.

Note that **rtla timerlat** was dispatched without changing *timerlat* tracer
threads' priority. That is generally not needed because these threads hava
--
2.39.1